The eye is a human organ and has been extensively studied, especially concerning its movements. FIG. 1 diagrammatically shows a front view of an eye. This eye comprises an eye-socket 1, the eyelid 2 being located above said eye-socket. Inside the orbit 1, it is possible to see a portion of the sclera 3 which forms a sort of white sphere having a circular opening where the cornea 4 is inserted, slightly more bulged than the sclera 3. The cornea 4 is transparent and thus makes it possible to see the iris 5 which is an opaque membrane pierced with a circular opening, namely the pupil 6. This figure also shows a corneal reflection 7, that is the reflection which can be seen on the cornea 4 when the eye is illuminated from the front.
As shall be seen later in the description, several corneal reflections may be visible on the pupil 6, the sclera 3 or on the iris 5 when several light sources illuminate the eye.
Normally, the measuring of the movement of an eye consists of determining the position of the pupil and a corneal reflection of the eye. However, when the eye is lit up without taking particular precautions, the images embodied may contain certain dark portions, such as the eyelashes and the eyebrows which merge with the dark disk of the pupil. Similarly, certain light portions of the embodied image, such as the reflections on the skin of the eyelids 2 or the white of the sclera 3, may be merged with the bright stain of the corneal reflection 7. These dark or light portions respectively distort the calculation of the center, pupil or the corneal reflection respectively.
The patent application FR-A-2 684 285 offers a method and device for improving the quality of the images to be treated and thus the precision of the measurements of the position of the eye determined on the basis of these images. This method consists of illuminating the eye, alternatively or simultaneously, by two light sources of distinct directions.
According to this document, the movements of the eye are measured in a mark linked to the head of the subject. This accordingly results in a slight constraint as regards the subject. In fact, in order to know the position of the gazer on a screen, it is therefore necessary to immobilize the head of the subject in front of the screen.
In addition, this method and device are unable to determine the position of the gaze point on a video screen in real time.
However, there are measuring systems able to determine the position of the fixing point of an eye on a video screen. One of these systems is described in the article entitled &lt;&lt;A study of human interface using an eye-movement detection system&gt;&gt; by M. IIDA, A. TOMONO and Y. KOBAYASHI in ATR Communications Systems Research Laboratories, Sanpeidani, Inuidani, Seika-cho, Souraku-gun, Kyoto 619-02 (Japan). This system has the drawback of requiring a total or partial immobilization of the head of the patient in front of the video screen or indeed requiring the addition of a system for measuring the movements of the head of the subject.